The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the devices, systems and/or methods described herein or the background. The disclosure of any reference cited herein is incorporated by reference.
In an electrochemical gas sensor, the gas to be measured typically passes from the atmosphere into the sensor housing through a gas porous or gas permeable membrane to a first electrode known as a working electrode (sometimes called a sensing electrode) where a chemical reaction occurs. A complementary chemical reaction occurs at a second electrode known as a counter electrode (or an auxiliary electrode). The electrochemical sensor produces an analytical signal via the generation of a current arising directly from the oxidation or reduction of the analyte gas (that is, the gas to be detected) at the working electrode. A comprehensive discussion of electrochemical gas sensors is also provided in Cao, Z. and Stetter, J. R., “The Properties and Applications of Amperometric Gas Sensors,” Electroanalysis, 4(3), 253 (1992), the disclosure of which is incorporated herein by reference.
To be useful as an electrochemical sensor, a working and counter electrode combination must be capable of producing an electrical signal that is (1) related to the concentration of the analyte gas and (2) sufficiently strong to provide a signal-to-noise ratio suitable to distinguish between concentration levels of the analyte gas over the entire range of interest. In other words, the current flow between the working electrode and the counter electrode must be measurably proportional to the concentration of the analyte gas over the concentration range of interest.
In addition to a working electrode and a counter electrode, an electrochemical sensor often includes a third electrode, commonly referred to as a reference electrode. A reference electrode is used to maintain the working electrode at a known voltage or potential. The reference electrode should be physically and chemically stable in the electrolyte.
Electrical connection between the working electrode and the counter electrode is maintained through an electrolyte. Functions of the electrolyte include: (1) to efficiently carry the ionic current; (2) to solubilize the analyte gas; (3) to support both the counter and the working electrode reactions; and (4) to form a stable reference potential with the reference electrode. Criteria for an electrolyte can include the following: (1) electrochemical inertness; (2) ionic conductivity; (3) chemical inertness; (4) temperature stability; (5) low cost; (6) low toxicity; (7) low flammability; and (8) appropriate viscosity.
In general, the electrodes of an electrochemical cell provide a surface at which an oxidation or a reduction reaction occurs to provide a mechanism whereby the ionic conduction of the electrolyte solution is coupled with the electron conduction of the electrode to provide a complete circuit for a current.
The measurable current arising from the cell reactions of the electrochemical cell is directly proportional to the extent of reaction occurring at the electrode. Preferably, therefore, a high reaction rate is maintained in the electrochemical cell. For this reason, the counter electrode and/or the working electrode of the electrochemical cell generally include an appropriate electrocatalyst on the surface thereof to support the reaction rate.
An electrochemical gas sensor in which two or more gas analytes are to be detected typically includes two or more working electrodes. Those electrodes can, for example, be placed in close proximity to each other (for example, to be adjacent and coplanar within the sensor) to provide a similar diffusion path from the inlet(s) of the sensor to each of the electrodes. Often, sensors that detect more than one analyte gas (which include more than one working electrode) can suffer cross-sensitivity of one analyte gas on at least one of the working electrodes that was designed to detect another analyte gas. One possible cause of this cross-sensitivity is lateral diffusion through a diffusion membrane and/or electrolyte to the adjacent electrode.
Several strategies have been used to address cross-sensitivity between two or more working (or other) electrodes. One strategy is the creation of a barrier to diffusion between electrodes by creating slots in a common or shared diffusion membrane between the catalysts of each electrode. The slot or slots are filled with liquid electrolyte when the sensor is filled with electrolyte. A similar approach is to place the electrodes on two separate diffusion membranes and provide for a gap between them that can likewise be filled with electrolyte to create a diffusion barrier. While the electrolyte will reduce gas diffusion, it will not completely eliminate it. In that regard, it is well known that gases will dissolve in electrolytes and migrate or diffuse therethrough, albeit at a reduced rate, than through a gas diffusion membrane used in connection with electrodes. Further, as it is often desirable to minimize the size of sensors, the distance that can be maintained between separate diffusion membranes or between separate catalyst layers on a common diffusion membrane is limited. Another approach is to compress the diffusion membrane in the area between the electrodes to create a “less permeable region”. Such compression can, for example, be achieved with a bar-like structure or abutment member (for example, formed in the sensor lid) that mechanically compresses the membrane when the sensor is assembled. Although lateral diffusion is reduced by such compression, it is not totally eliminated.